The present invention is a process for the organooxylation of chlorosilanes. The process comprises contacting in a film a chlorosilane and an alcohol capable of forming an ester with the silicon of the chlorosilane and thereby forming an equilibrium mixture comprising an organooxysilane and hydrogen chloride. The film is heated at a temperature sufficient to cause vaporization of the hydrogen chloride from the equilibrium mixture thereby increasing the yield of organooxysilane in the equilibrium mixture. In a preferred process, the process is run in a falling-film type reactor or a wiped-film type reactor.
Organooxysilanes prepared by the present process are useful intermediates in the production of other silanes and may be used as coupling agents, silylation agents, and surface treatments. The organooxysilanes are generally safer to handle than the chlorosilanes from which they are formed and the organic substituent of the organooxy group can be tailored to make the organooxysilanes more compatible with organic compounds and materials.
The reaction of an alcohol with a chlorosilane to form an organooxysilane is an equilibrium reaction exemplified by the following equation: EQU =Si-Cl+ROH&lt;-&gt;=SiOR+HCl
Therefore, to drive the reaction to favor high yields of the organooxysilane it is desirable to remove the hydrogen chloride as it is formed from the reaction mixture. In addition the hydrogen chloride liberated during the reaction can attack the starting materials and products to produce undesirable by-products which also lowers the yield of the desired products. For example, liberated hydrogen chloride can react with alcohol to produce a hydrocarbon chloride and water. This results in the loss of considerable alcohol. Furthermore, water formed by this side reaction can hydrolyze the chlorosilane producing undesirable polysiloxanes and generating more hydrogen chloride. In addition the hydrogen chloride alone or in combination with the alcohol may react with other functional groups present on the chlorosilanes.
Therefore it is an objective of the present invention to provide a process where hydrogen chloride liberated during the process is rapidly and effectively removed from the reaction mixture. The present inventors have found that this objective can be achieved by running the described equilibrium reaction in a thin-film process. The present process provides an effective means for removing reaction-liberated hydrogen chloride from the process and thereby shifting the chemical equilibrium of the process to favor production of organooxysilanes and also a means for minimizing side reactions and undesired by-products as a result of these side reactions. It is furthermore an objective of the present invention to provide a process with improved mass transfer thereby allowing for more efficient reactor operation than is achieved with reactive-distillation type reactors in which the reaction is typically conducted on a commercial scale.
Various processes have been reported in the art to shift the chemical equilibrium of the reaction of an alcohol with a chlorosilane and to reduce unwanted side reactions.
Schubert, U.S. Pat. No. 3,008,975, issued Nov. 14, 1961, describes a batch process for reacting a chlorosilane with an alcohol. Schubert reports that at reactor pressures below about 200 mm Hg side reactions are reduced and process yield is improved.
Bennett, U.S. Pat. No. 3,651,117, describes a process for reacting a chlorosilane with an alcohol where the chlorosilane, alcohol, and resultant product are kept in the vapor state. Bennett reports this process results in increase process yield and reduced side reactions. Bennett also reports that it may be useful to include in the reaction mixture a base such as ammonia or an amine for the purposes of sequestering hydrogen chloride, to purge the reaction mixture with a gas to remove hydrogen chloride, or to react the reaction mixture with a vicinal alkylene oxide to remove hydrogen chloride.
Nitzsche et al., U.S. Pat. No. 3,792,071, issued Feb. 12, 1974, describe the use of a fractional distillation column for the reaction of an alcohol with a chlorosilane. The chlorosilane is introduced at the head of the column and the alcohol is introduced in the gaseous form from below or at a point in the lowest one-third of the length of the column. For at least two-thirds of the zone between the inlet of the alcohol and the inlet of the silane into the column the column is maintained over its entire internal cross-section at a temperature at least 0.5.degree. C. above the boiling point of the particular alcohol used.
Kotzsch et al., U.S. Pat. No. 3,985,781, issued Oct. 12, 1976, report a two-step batch process where in the first step a primary alcohol is contacted with a trichlorosilane sufficient to effect only partial esterification of the trichlorosilane. The hydrogen chloride is then removed from the reactor by heating and then additional alcohol is added to the reactor to complete the esterification process.
Kotzsch et al., U.S. Pat. No. 4,039,567, issued Aug. 2, 1977, describe a process for reacting a chlorosilane with an alcohol in a distillation column. The process comprises feeding a liquid alcohol and a liquid chlorosilane into a distillative reaction zone having a head portion and a sump portion. The head portion is maintained at a temperature sufficient for the esterification reaction to occur and gaseous hydrogen chloride formed by the reaction is continuously distilled off.
Seiler et al. U.S. Pat. No. 4,173,576, issued Nov. 6, 1979, describe an improved process for the esterification of chlorosilanes with alcohols. The esterification process is carried out in the presence of a chlorohydrocarbon and in the absence of an acid binding agent. Seiler et al. state that it has been proposed to purge out the hydrogen chloride forming as a result of the process by passing an inert gas over the surface or through the reaction mixture with the aid, in some cases of a falling film evaporator. Seiler et al. conclude that this in an impractical approach due to the quantity of exhaust gas created, loss of product, and need for large capacity cooling apparatuses.
Schinabeck et al., U.S. Pat. No. 4,298,753, issued Nov. 3, 1981, describe a continuous two-stage process for preparing alkoxysilanes. The process comprises introducing in a liquid phase a chlorosilane and a hydroxyl-containing aliphatic compound in parallel flow into a first reactor; then removing the liquid reaction mixture from the first reactor and introducing it at the head of a column used as the second reactor, which is maintained at an elevated temperature, and adding a hydroxyl-containing aliphatic compound as a gas at the lower end of the column. An alkoxysilane product is recovered from the bottom of the column.
Fischer et al., U.S. Pat. No. 4,506,087, issued Mar. 19, 1985, teach a continuous process for preparation of alkoxysilanes with hydrogen chloride contents of less than 20 ppm. In the described method, the esterification is performed in a reaction vessel and the raw esterification product is continuously removed and delivered to the top of a column. In this column, the reactant alcohol is vaporized and condensed at the top. The raw product drips from the top of the column to the bottom where it is collected.
Bank et al., U.S. Pat. No. 4,924,022, issued May 8, 1990, teach a continuous system for the manufacture of organoalkoxysilanes. The reactor consists of a fractionating column that allows for completion of the reaction and separation of the hydrogen chloride formed as a by-product.
The cited art does not recognize that the esterification reaction of alcohol with a chlorosilane can be conducted as a thin-film process thereby effecting efficient vaporization and removal of hydrogen chloride from the process. The present process can increase yield of organooxysilane product and reduced undesired side reactions. The present process can also provided for comparable yields in simpler and smaller reactors, when compared to typical reactive distillation processes in current commercial use.